Add support for PyPy on Windows

[giampaolo]

After all this time it turns out PyPy on Windows was not supported since who knows when (could not compile due to some missing Python.h definitions). PyPy on Windows seems to have some issues:

• process exe() / NTQuerySystemInformation: can fail with ERROR_TOO_MANY_OPEN_FILES. Perhaps because PyPy JIT delays garbage collection of files? Anyway, we now force and raise AcessDenied
• segfault if passing invalid unicode strings to process environ() or when 3 suprocesses are created in a row